A wheel rim 1 constituting a wheel of an automobile and a rotor 2 constituting a disk brake functioning as a damping device are rotatably supported by a knuckle 3 constituting a suspension device, for example, in the structure as illustrated in FIG. 1 showing a first example of the embodiment of the present invention. That is, an outer race 6 constituting a stationary member which does not rotate even in use and constitutes a wheel bearing unit 5 is fixed by a plurality of connection bolts 7 to the circular supporting hole 4 formed through this knuckle 3. On the other hand, the wheel rim 1 and the rotor 2 are securely connected by a plurality of studs 9 and nuts 10 to a hub 8 constituting a rotary member which rotates in use and constitutes the wheel bearing unit 5.
The outer race 6 is formed respectively with double-row outer raceways 11a and 11b as stationary raceways in the inner peripheral surface thereof and with a connection flange 12 on the intermediate portion of the outer peripheral surface thereof. The outer race 6 in this configuration is fixed to this knuckle 3 by connecting this connection flange 12 to the knuckle 3 with the respective connection bolts 7. On the other hand, an attachment flange 13 is formed on a portion of the outer peripheral surface of the hub 8 extending beyond the outer opening of the outer race 6 (the term “outer” in the axial direction is used to designate the side located outwardly in the width direction of a vehicle in which the unit is assembled, i.e., the left hand side of the respective figures, and conversely the term “inner” in the axial direction is used to designate the side located inwardly in the width direction of a vehicle in which the unit is assembled, i.e., the right hand side of the respective figures).
The wheel rim 1 and the rotor 2 are securely connected to an attachment surface 14 in one side surface (the outer side surface in the illustration) of this attachment flange 13. For this purpose, a number of attachment holes 15 are formed in a plurality of positions arranged in the circumferential direction of the attachment flange 13 and located on the same circle having its center at the central axis of the hub 8 in order to pass through the opposite side surfaces of the attachment flange 13 respectively. Also, each of the respective studs 9 is formed respectively with a serration section 16 in the outer peripheral surface of the base end portion (the right end of FIG. 1) thereof and with a male thread section 17 (refer to FIGS. 2 to 4 to be described below) in the outer peripheral surface of the tip half portion (the left half section of FIG. 1) thereof. Then, the base end portion of each of the respective studs 9 is fixed to the attachment flange 13 by the press-fit insertion of the serration section 16 thereof into the corresponding attachment hole 15 in order to inhibit the rotation of the respective studs 9 relative to this attachment flange 13.
The rotor 2 is arranged to overlap with the attachment surface 14, and furthermore the wheel rim 1 is arranged to overlap with one side surface of this rotor 2. In this condition, the tip portion of the respective studs 9 is inserted through the through holes 18 and 19 opened respectively through the wheel rim 1 and the rotor 2 in order to project from one side surface of the wheel rim 1. Then, the male thread section 17 is threaded into and fastened to the nut 10 at the tip portion of the stud 9 projecting from the one side surface of this wheel rim 1. By this configuration, the wheel rim 1 and the rotor 2 are securely connected to the attachment surface 14 of the attachment flange 13.
Also, the hub 8 is configured by combining a hub body 20 and the inner race 21. Thereamong, the hub body 20 is provided with a first inner raceway 22 as a rotary side raceway which is directly formed on the hub body 20 in the outer peripheral surface of the intermediate portion opposite the outer raceway 11a among the double-row outer raceways 11a and 11b. Furthermore, the inner race 21 is securely fitted onto a small diameter stepped portion 23 formed on the outer peripheral surface of this hub body 20 at the inner end to form the hub 8. Then, a second inner raceway 24 serving also as a rotary side raceway is formed on the outer peripheral surface of this inner race 21 in a location opposite the outer raceway 11b which is the inner one of the double-row outer raceways 11a and 11b. Also, in the case as shown in the figure, this inner race 21 is fixed to the hub body 20 by holding the inner end surface of the inner race 21 with a crimped section 25 formed by plastically deforming the inner end of the hub body 20 outwardly in the radial direction.
Furthermore, a plurality of balls 26 and 26, which are rolling members respectively, are rollingly supported between each of the outer raceways 11a and 11b and the corresponding one of the first and second inner raceways 22 and 24 with retainers 27 and 27 holding these balls. By this configuration, a double-row angular type ball bearing is formed in a back-to-back combination fashion in which the hub 8 is rotatably supported in the inner side of said outer race 6 to receive a radial load and a thrust load. Incidentally, the seal rings 28a and 28b are interposed between each of the opposite ends of the inner peripheral surface of the outer race 6 and the corresponding one of the outer peripheral surface of the intermediate portion of the hub 8 and the outer peripheral surface of the inner end of the inner race 21 in order to isolate the inner space accommodating the respective balls 26 and 26 from the external space. Furthermore, since the wheel bearing unit 5 in the example as illustrated is for use in a driven wheel (the rear wheels of an FR or RR vehicle, the front wheels of an FF vehicle, or all the wheels of a 4WD vehicle), a spline hole 29 is formed in the central location of the hub body 20. And, a spline shaft 31 provided on a constant velocity joint 30 is inserted into this spline hole 29.
When the wheel rolling bearing unit 5 as described above is used, as illustrated in FIG. 1, the outer race 6 is fixed to the knuckle 3 while the wheel rim 1 supporting a tire (not shown in the figure) and the rotor 2 as a rotatable member for braking are fixed to the attachment flange 13 of the hub 8. Among them, the rotor 2 is combined with a support and a caliper fixed to the knuckle 3 (not shown in the figure) to form a disk brake for braking. During a braking operation, a pair of pads located with the rotor 2 placed between them is urged against the opposite side surfaces of this rotor 2 which are braking friction surfaces. In the description herein, the term “braking friction surface” is intended to mean, in the case where the rotatable member for braking is a rotor, the opposite side surfaces in the axial direction of this rotor and, in the case where the rotatable member for braking is a drum, the inner peripheral surface of this drum.
Meanwhile, it is known that a vibration with unpleasant noise, as referred to as judder, often occurs during automotive braking. While several causes are known as the causes of such a vibration, for example, the nonuniform friction state between the side surface of the rotor 2 and the lining of the pads, the runout of the rotor 2 is known as a major cause. Namely, while the side surface of this rotor 2 has ideally to be assembled normal to the center of rotation of this rotor 2, it is difficult to perfectly realize the normal orientation due to manufacturing error which is inevitable. As a result, when driving an automobile, the runout of the side surface of the rotor 2 can not be avoided, although being more or less, in the direction of the rotatable shaft (in the right and left direction in FIG. 1).
If such a runout (the amount of displacement in the right and left direction in FIG. 1) increases, the above judder occurs when the lining of the pair of pads is urged for braking against the opposite side surfaces of the rotor 2. Also, in the case where the drum constituting a drum brake is fixed to the side surface of the attachment flange 13, a vibration such as judder occurs when a shoe is urged against this inner peripheral surface of the drum unless the inner peripheral surface of this drum is perfectly in parallel with the center of rotation of the drum. In order to inhibit the judder as generated by such a cause, it is important to suppress (or prevent) the runout (axial runout) of the side surface of the rotor 2 in the axial direction or to suppress (or prevent) the runout of the inner peripheral surface of the drum in the radial direction.
However, in the case of a conventional wheel bearing unit, for the following reasons, there is the possibility that the runout of the braking friction surface (the side surface of the rotor 2 or the inner peripheral surface of the drum) becomes likely.
At first, in the case of the so-called third-generation wheel bearing unit 5 as illustrated in FIG. 1 in which the first inner raceway 22 is formed directly in the outer peripheral surface of the intermediate portion of the hub 8, it becomes difficult to secure the parallelism of the first and second inner raceways 22 and 24. Namely, in the case of the so-called second-generation wheel bearing unit in which a pair of inner races are securely fitted onto the hub body, the outer peripheral surface of the small diameter stepped portion, long in width, formed on the hub body for externally fitting and fixing the pair of inner races thereto is a single cylindrical surface whose diameter is limited to not changed over the almost entire length of the portion onto which both the inner races are securely fitted. It is relatively easy to machine such a single cylindrical surface, and therefore when the first and second inner raceways are provided on the outer peripheral surface of the hub by fitting and fixing the pair of inner races having the same diameter onto the small diameter stepped portion, the parallelism of the two inner raceways can be easily secured with reference to rotation center as long as the precision of the pair of inner races can be secured.
Contrary to this, in the case of the third-generation wheel bearing unit, there is formed a step between the portion of the axially intermediate portion of the hub body 20 in which the first inner raceway 22 is formed, and the small diameter stepped portion 23 for externally fitting and fixing the inner race 21 for providing the second inner raceway 24. Accordingly, as compared with the second-generation wheel bearing unit, there is the possibility that the braking friction surface of the rotatable member for braking is subject to runout due to the degradation of the parallelism of the first and second inner raceways 22 and 24.
Also, in the case as shown in FIG. 1 in which the inner race 21 is fixed to the hub body 20 by holding the inner end surface of the inner race 21 with the crimped section 25 formed by plastically deforming the inner end of the hub body 20 outwardly in the radial direction, there is the possibility that the attachment flange 13 is slightly deformed when this crimped section 25 is formed depending upon the fixation method of this hub body 20. Namely, when the crimped section 25 is formed, a large load is applied to the inner end of the hub body 20 in order to plastically deform this inner end with the attachment flange 13 being supportingly fixed. Because of this, on the basis of this load, there is the possibility that the attachment surface 14 provided in one side surface of the attachment flange 13 is slightly deformed and that the braking friction surface of the rotatable member for braking which is overlappingly fixed to this attachment surface 14 is subject to runout.
Furthermore, in the case of the structure as illustrated in FIG. 1, the plurality of the studs 9 for supporting the rotatable member for braking are press-fitted and fixed into the respective attachment holes 15 formed in the attachment flange 13. Because of this, during the press-fit insertion, there is the possibility that the opening edge peripheral portion of the respective attachment holes 15 which is a portion of the attachment surface 14 (the circular ring portion within 2.5 mm outwardly in the radial direction from the periphery of the opening of the respective attachment holes 15) is slightly projected in the direction to form the convex portion as viewed from the attachment surface 14 by virtue of the engagement of the inner peripheral surface of this attachment hole 15 with the serration section 16 of the stud 9. Particularly, if the amount of deformation of the opening edge peripheral portion 15 varies from one to another of the respective attachment hole 15, it is undesirable because the braking friction surface of the rotatable member for braking overlappingly connected and fixed to the attachment surface 14 is subject to runout.
In the case where the first inner raceway 22 is directly formed on the hub body 20 with the crimped section 25 formed at the inner end of this hub body 20, the braking friction surface of the rotatable member for braking is subject to runout, and therefore it is particularly important to inhibit the runout of the braking friction surface due to the press-fit insertion of the stud 9 as described above.
Taking into consideration the above circumstances, the wheel bearing unit in accordance with the present invention has been made in order to realize the structure for inhibiting the runout of the braking friction surface due to the press-fit insertion of studs and preventing the occurrence of judder due to the runout of this braking friction surface.